The present invention relates to a helically wound forming tube for use in a method of forming glass fiber and to a method of forming glass fiber.
Although the process of making glass fiber will be explained in somewhat more detail hereinafter, such a process can basically be summarized as involving the winding of a glass fiber strand around a rotating forming tube which must be both strong and flexible. After the winding is completed, the glass fiber is further processed, and then the forming tube is partially collapsed and extracted from the interior of the circular glass fiber windings so that the strand can be unwound by grasping the interior lead end. Because of the method of processing glass fiber and the types of steps employed, the forming tube must have good wet strength and be resistant to both heat and centrifugal forces.
Prior art forming tubes comprise a helically wound tube employing three or more plies of kraft paper, each ply containing a spiral butt joint, i.e., each edge of the ply, or strip of paper, in each ply, or layer, abuts the adjacent edge. The spiral butt joints were staggered for strength purposes.
These prior art forming tubes are made by helically winding separate plies of paper around a stationary mandrel as is well known in the art and represented by U.S. Pat. Nos. 3,165,034 and Re23,899. The forming tube was then conventionally treated with a silicone release agent to enable the tube to be more easily removed from the interior of the glass fiber strand circularly wound therearound.
A forming tube when used in forming glass fiber is located on a collet drive and rotated about the axis of the tube. The tube is brought up to speed before winding of the glass fiber commences. Heretofore, rotation speeds of 3,000-4,000 rpm have been used. Problems have arisen in the formation of glass fiber using more modern apparatus which has higher speeds. As the speed of the rotary winding step is increased, a stronger forming tube must employed. In fact, prior art tubes have not been usable at high speeds because the increased centrifugal forces cause them to rupture. However, the forming tube must still maintain the needed flexibility in order to be withdrawn from the center of the wound glass fiber and in order to maintain a low unit per use cost.
The same type of general process for forming helically wound tubes has been used to make rigid textile carriers such as that represented by U.S. Pat. No. 2,751,936 which has three inner plies of spiral butt joints and one outer ply with a spiral overlapped joint. The purpose of the spiral overlapped joint is to enable the tube to take on a smooth outer surface after the outer ply of a very thin paper was sanded or skived and treated with various compositions.
It is also known to provide spiral overlapped joints on both the inner and outer plies for various purposes such as mailing tubes, U.S. Pat. No. 726,894 and food containers, 3,183,802. U.S. Pat. No. 2,181,035 discloses spiral overlapped joints for intermediate and outer plies with an inner ply having a spiral butt joint for tubing used to insulate electrical conductors. The tube is stated to have an increased tensile strength with sufficient flexibility to be bent, twisted or collapsed without objectionable injury in order to achieve the desired accordian flexure of the plies required for insulating electrical conductors. These characteristics are stated to be achieved by providing at least one layer of a cellophane-type material having overlapped spiral joints and one or more layers of kraft paper together with one or more layers of crepe paper which also may have overlapped spiral joints. However, this patent relates to a tube for a totally nonanalogous use where accordion-type flexure is desired; furthermore, such a tube is not used under conditions where the tube is intentionally treated with an aqueous spray and then subjected to relatively high temperatures as in a glass fiber forming process. Additionally, this patent does not disclose the structure of the present invention whereby the spiral joint of the inner ply is reinforced by stacking the spiral overlapped joint of the outer ply thereabove.